Sighted popping up next to various train windows at very close range, only to disappear when someone inside tried to alert the other passengers. It is thought that the arms are their attempt to copy human biology, though it clearly didn't work well. As for the bushes, they're probably thrusters...

Nice, the mimicking theme is what I was going after with my Mysterionicles. You got right the notion that you can't judge mysterions' parts by their look :) Plus, they're easy to scale, 'cause they're usually minifig scale (the canopy is supposed to carry a pilot or a CPU, or maybe even BE a crature... as long as nobody spoils it lol)